The group of five young British men were released on bail Monday and counterterror detectives said they were investigating “multiple lines of inquiry.”
US Secretary of State Marco Rubio has said an incident involving the arrest of five men on explosives and terror charges near an RAF base in southern England “clearly involves the hands of a foreign actor” but did not name a specific country.
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Counter-terrorism police said on Monday they were investigating whether there was any link between the incident and Iran.
“What could have happened in the UK over the weekend is a very serious situation,” Rubio said on Fox News on Monday evening, but provided no evidence for his claims that another country was involved.
Iran's embassy in London said in a post on X that it "categorically rejects and strongly condemns" the "unfounded and malicious speculations" that Tehran was involved.
The group of five young British men were released on bail Monday and counter-terror detectives said they were investigating “multiple lines of inquiry.”
The men were detained early on Sunday after three vans were spotted in the countryside near RAF Fairford, a UK-owned base that has been used by American bombers during decades of conflicts in the Middle East, including this year's war with Iran.
Investigators haven't concluded whether the incident was state-backed. They are investigating a possible link to Iran, Russia or non-state militant groups. RAF Fairford has also been the focus of protests by anti-war campaigners.
The five suspects, who haven't been named, were initially held over suspected explosives offenses and later also arrested on suspicion of preparing an act of terrorism.
Police said the men are all British nationals from London and in their 20s.
Britain's national news agency, PA, reported without citing sources that police had not found viable explosives.
Announcing that the suspects had been bailed, the head of UK Counter Terrorism Policing, Laurence Taylor, said that the suspects face stringent conditions and “absolutely remain under investigation."
Taylor said lines of inquiry include that the suspected offenses were committed by "proxies or individuals, either knowingly or unknowingly, working on behalf of a foreign state.”
Residents were evacuated from 85 homes in the village of Whelford near the base on Sunday and army bomb-disposal experts used a robot to examine three white vans.
The villagers were allowed to return home late Monday afternoon, though a cordon remained around the three vehicles, police said.
Prime Minister Andy Burnham chaired a meeting of the government’s COBRA emergency committee on Monday “to make sure we have the full picture" of the incident, he said.
Arrests came after reports of suspicious vehicles
It was unclear whether the arrested men had been on security services' radar. Local residents described seeing heightened security around RAF Fairford in recent days, with heavy vehicles parked across entrance gates.
US President Donald Trump said the suspects had been detained following cooperation between Washington and London and had been “under view” for some time.
Police, however, said the arrests were triggered by a late-night call about “suspicious vehicles” near the base.
A local farmer said she called police after seeing three oddly parked vans as she drove home late Saturday night, and that nearby she came across “a large group of men,” some with their faces covered. She said they scattered when they saw her vehicle.
She declined to be identified to protect her identity because she was describing what she saw as potential security lapses.
British Defence Secretary Wes Streeting said the farmer “did exactly the right thing” by calling police but only had “a partial view” of the operation.
The arrests came at a time of heightened tensions around the threat from hostile states, with British intelligence services saying Russia and Iran have both used proxies to commit crimes on UK soil.
In March, officials said the number of national security cases involving activity by hostile states, including Iran, increased by 50% in the six months to December 2025.
Iran linked to series of attacks in Europe
Britain didn't join the US and Israel in the Iran war, which began on 28 February, and initially denied Trump’s request to use bases in the UK to strike the country.
But then-Prime Minister Keir Starmer agreed in March to let the US use bases in Britain for what he called “defensive” operations against Iran’s missiles and their launch sites following Iranian attacks on UK interests and Britain’s allies in the Gulf.
American B-1 and B-52 bombers have taken off from Fairford, 160 kilometres west of London, for operations during the Iran conflict.
Iran has said that it considers any base used to attack its territory a legitimate target.
British security officials say proxies linked to Iran have been behind a string of arsons and other attacks in the UK and other countries, many of them targeting the Jewish community and intended to sow fear.
They have previously warned that Tehran could expand its targets in the UK if conflicts in the Middle East continue.
UK Foreign Secretary Ed Miliband said on Monday that the incident is a “reminder of the threats our country faces.”